Method of producing epitaxial silicon wafer

ABSTRACT

Provided is a method of producing an epitaxial silicon wafer, which is excellent in productivity and prevents the formation of a backside haze in consecutive single-wafer processing epitaxial growth procedures on a plurality of silicon wafers without cleaning a process chamber after each epitaxial growth procedure. The method of producing an epitaxial silicon wafer includes: a step of loading a silicon wafer; a step of forming a silicon epitaxial layer; a step of unloading the silicon wafer; and a cleaning step. The cleaning step is performed before and after repeating a predetermined number of times a series of growth procedures including the silicon wafer loading step, the silicon epitaxial layer formation step, and the silicon wafer unloading step.

TECHNICAL FIELD

This disclosure relates to a method of producing an epitaxial siliconwafer, and in particular relates to a method of producing an epitaxialsilicon wafer, in which a plurality of silicon wafers are subjected torepeated epitaxial growth procedures without cleaning a process chamberin a single-wafer processing epitaxial growth apparatus after eachprocess.

BACKGROUND

Epitaxial silicon wafers obtained by epitaxially growing a siliconepitaxial layer on a silicon wafer are used as device substrates forfabricating various types of semiconductor devices such as metal oxidesemiconductor field-effect transistors (MOSFETs), dynamic random accessmemories (DRAMs), power transistors, and back-illuminated solid-stateimaging devices.

Epitaxial growth on a silicon wafer is typically performed using asingle-wafer processing epitaxial growth apparatus. FIG. 1 depicts atypical single-wafer processing epitaxial growth apparatus. Thissingle-wafer processing epitaxial growth apparatus includes a load-lockchamber 100 with a nitrogen atmosphere, a transfer chamber 200 with anitrogen atmosphere, a process chamber 300 with a hydrogen atmosphere,and a storage enclosure 400. A series of epitaxial growth procedures isdescribed with reference to FIG. 1.

The storage enclosure 400 and the load-lock chamber 100 are connectedusing a gate valve 50. The gate valve 50 is opened to unload a siliconwafer 10 from the storage enclosure 400 to the load-lock chamber 100.The gate valve 50 is closed again. Further, a gate valve 150 between theload-lock chamber 100 and the transfer chamber 200 is opened to removethe silicon wafer 10 from the load-lock chamber 100 using a transferunit (not shown). Further, the silicon wafer 10 is transferred to thetransfer chamber 200, and the gate valve 150 is closed.

Next, when a gate valve 250 between the transfer chamber 200 and theprocess chamber 300 is opened, the silicon wafer 10 is transferred fromthe transfer chamber 200 to the process chamber 300 using the transferunit. When the silicon wafer 10 is placed on a susceptor 310 in theprocess chamber 300, the gate valve 250 is closed. After that, thesilicon wafer 10 is subjected to epitaxial growth in the process chamber300, and a silicon epitaxial layer 20 is formed on the silicon wafer 10,thus an epitaxial silicon wafer 1 is obtained.

The resultant epitaxial silicon wafer 1 is transferred from the processchamber 300 to the load-lock chamber 100 via the transfer chamber 200through the above series of growth procedures in the reverse order tothe order of the above described series of growth procedures, and isthen stored in a wafer storage enclosure. A single-wafer processingepitaxial growth apparatus is usually provided with, in addition to theload-lock chamber 100 and the storage enclosure 400 for removing thesilicon wafer 10 to be subjected to epitaxial growth (hereinafter alsoreferred to as “film deposition”), a load-lock chamber and a storageenclosure for storing epitaxial silicon wafers 1 having been subjectedto film deposition. Note that the load-lock chamber and the waferstorage enclosure for storing are not shown in FIG. 1 for brevity.

For example, JP H11-186363 A (PTL 1) discloses a semiconductorprocessing apparatus having a load-lock chamber including a storageenclosure for storing semiconductor wafers, which chamber is used toload and unload semiconductor substrates; a process chamber forprocessing the semiconductor substrates; and a transfer chamber which isconnected to the load-lock chamber and the process chamber withrespective gate valves and is used to transport the semiconductorsubstrate between the adjacent chambers.

Here, when epitaxial growth is performed, silicon is deposited to form asilicon epitaxial layer on a silicon wafer surface; however, silicon isalso deposited on the inner wall surface of the process chamber, asusceptor, etc. This being the case, after the epitaxial growth, theinside of the process chamber is cleaned by vapor phase etching of thedeposit using for example hydrochloric gas.

The methods of cleaning the process chamber in epitaxial growthperformed using an epitaxial growth apparatus can be roughly dividedinto the following two categories. A first method is a method in whichthe inside of the process chamber is cleaned each time an epitaxialgrowth process is performed (hereinafter referred to as “single depoprocess”). A second method is a method in which an epitaxial growthprocess is repeated on a plurality of silicon wafers without cleaningthe process chamber after each epitaxial growth process (hereinafterreferred to as “multi depo process”).

Differences between the single depo process and the multi depo processare further described using the examples of Process A and Process Billustrated in FIG. 2. In the single depo process, as in Process Aillustrated in FIG. 2, a cleaning procedure C1 of the process chamber isperformed first, and an epitaxial growth procedure E1 is then performedon a first silicon wafer loaded into the process chamber through aload-lock chamber and a transfer chamber from a storage enclosure, thusa first silicon epitaxial wafer is formed. Next, the silicon epitaxialwafer formed is unloaded through the transfer chamber from the processchamber to a load-lock chamber and a storage enclosure for storing inorder. After a cleaning procedure C2 for the process chamber isperformed, a second silicon wafer is loaded into the process chamber andsubjected to an epitaxial growth procedure E2, thus a second siliconepitaxial wafer is formed. Such cleaning and epitaxial growth proceduresare sequentially repeated to form five silicon epitaxial wafers in theexample of Process A in FIG. 2.

Epitaxial growth procedures E1, E2, . . . , and E5 include heating,hydrogen bake-out, film deposition, and cooling, and the processes areindicated by the process temperatures in Process A in FIG. 2. The samecharacters are also used for Process B. Further, cleaning procedures C1.C2, . . . and C6 include a heating process, a hydrogen bake-out process,a vapor phase etching process, a Si coating process on a susceptor, anda cooling process, and as in the epitaxial growth procedures, theprocesses of the cleaning procedures are indicated by the processtemperatures in Process A of FIG. 2. The same applies to cleaningprocedures C1′ and C2′ in Process B.

On the other hand, in the multi depo process, the cleaning procedure C1′for the process chamber is performed first as illustrated in Process Bof FIG. 2. A silicon wafer is then loaded into the process chamber, andsubjected to the epitaxial growth procedure E1, thus an epitaxialsilicon wafer is formed. The epitaxial silicon wafer formed is unloadedthrough the load-lock chamber and the storage enclosure for storing inorder; however, the next silicon wafer is loaded into the processchamber without performing cleaning. Thus, a plurality of silicon wafersare subjected to repeated epitaxial growth procedures without cleaningafter each epitaxial growth procedure, and after performing theepitaxial growth procedure a predetermined number of times (five timesin Process B of FIG. 2), the cleaning procedure C2′ for the processchamber is performed (the cleaning procedure C2′ may also serve ascleaning for the next depo process of the multi depo process). Note thatthe above predetermined number of times, that is, the number of theepitaxial growth procedures repeated without cleaning in the multi depoprocess is hereinafter referred to as “multi depo number”.

In the single depo process, silicon deposit in the process chamber canbe eliminated each time an epitaxial growth procedure is performed, sothat stable quality can be achieved as compared with the multi depoprocess. This is because silicon deposit in the process chamberincreases as each epitaxial growth procedure is performed as describedabove.

On the other hand, in the multi depo process, as can be seen from thecomparison of the elapsed time between Process A and Process Billustrated in FIG. 2, the processing time of each cleaning procedure islonger than that in the single depo process. However, when the samenumber of epitaxial silicon wafers are formed, the total processing timeis usually shorter in the multi depo process than in the single depoprocess, thus the multi depo process is excellent in productivity.Accordingly, one of those processes is used depending on thespecification of the epitaxial silicon wafer to be formed. Note that theprocessing time of a cleaning procedure is longer in the multi depoprocess because the amount of silicon deposit is increased while aplurality of epitaxial growth procedures are performed unlike in thesingle depo process.

JP 2013-123004 A (PTL 2) describes that when the multi depo process isperformed many times, a so-called backside haze would easily be formedon the backside surface of an epitaxial silicon wafer in the latter halfof the multi depo process to cause poor appearance. In PTL 2, theformation of a backside haze is prevented in the multi depo process bycontrolling the heating process in the process chamber in the hydrogenbake-out step.

CITATION LIST Patent Literature

PTL 1: JP H11-186363 A

PTL 2: JP 2013-123004 A

SUMMARY Technical Problem

Since an epitaxial silicon wafer having a backside haze is regarded as adefective product, when the multi depo process is used, the multi deponumber is required to be set within the range where the backside hazedescribed above is not formed. In order to improve the productionefficiency for the epitaxial silicon wafer, it is preferred in terms ofimproving the productivity, that the maximum number of times the multidepo process can be repeated (hereinafter “maximum multi depo number”)is increased, and the multi depo process is performed the maximum multidepo number of times because the productivity can be improved. Althoughthe technique described in PTL 2 can increase the maximum multi deponumber, there is still room for improvement.

First, since the maximum multi depo number varies depending on theconditions such as apparatus specifications and epitaxial growthconditions in the multi depo process, the multi depo number needs to beconsidered and adjusted as occasion demands. A multi depo number that issmaller than an expected maximum multi depo number by a degree whichdoes not cause a backside haze can almost ensure the formation of abackside haze to be prevented; however, such a number results ininsufficient productivity.

It could therefore be helpful to provide a method of producing anepitaxial silicon wafer, which is excellent in productivity and preventsthe formation of a backside haze in repeating single-wafer processingepitaxial growth procedures on a plurality of silicon wafers withoutcleaning a process chamber after each epitaxial growth procedure.

Solution to Problem

We made various studies to achieve the above objective. We first studiedthe cause of the formation of a back surface in the latter half of theprocess of the multi depo process whereas no backside haze is formed inan early stage of the process when a plurality of silicon wafers aresubjected to repeated single-wafer processing epitaxial growthprocedures without cleaning a process chamber after each epitaxialgrowth procedure, namely, subjected to the multi depo process. Here, itis conceivable that the formation of a backside haze is caused on anatural oxide layer left unreduced by hydrogen gas on the back surfaceof a silicon wafer, due to mass transfer or etching caused by a reactionbetween a gas containing chlorine (Cl) that reaches the back surface ofthe silicon wafer and silicon on the susceptor.

In the process of the multi depo process, a load-lock chamber and atransfer chamber are kept flushed with nitrogen gas by nitrogen gaspurging in order to maintain a nitrogen atmosphere in the chambers.However, when a silicon wafer is loaded into the transfer chamber fromthe load-lock chamber, residual oxygen from the atmosphere, albeitslight, is considered to be flown into the transfer chamber. Further, inthe transfer chamber, oxygen from a natural oxide layer with a thicknessof several nanometers formed on a bare silicon surface of the siliconwafer is also present.

Although the transfer chamber is kept flushed with nitrogen gas, suchoxygen is considered to gradually accumulate and build up in thetransfer chamber as the process of the multi depo process proceeds. Forexample, as the process proceeds in the latter half of the multi depoprocess, oxygen is presumed to accumulate for example through adsorptionand desorption to/from the inner surface of the transfer chamber. Weconcluded that such oxygen accumulation in the transfer chamber, albeitslightly, increases the thickness of the natural oxide layer and wouldresult in the formation of a backside haze in the latter half of themulti depo process.

Against this backdrop, we changed the number of nitrogen flushingoperations on the transfer chamber focusing on the evacuation rate ofoxygen in the transfer chamber, and found that the maximum multi deponumber, that is, the maximum number of times the multi depo process canbe performed varied. We made further studies and found that the multidepo number greatly depends on the above number of purge operations.This discovery leads to this disclosure.

Specifically, this disclosure primarily includes following features.

(1) A method of producing an epitaxial silicon wafer, which epitaxiallygrows a silicon epitaxial layer on a silicon wafer, comprising:

a first step of loading a single silicon wafer from a first load-lockchamber into a transfer chamber kept purged with nitrogen gas;

a second step of loading the single silicon wafer from the transferchamber into a process chamber;

a third step of forming a silicon epitaxial layer on a surface of thesingle silicon wafer in the process chamber;

a fourth step of unloading the single silicon wafer on which the siliconepitaxial layer is formed, from the process chamber to the transferchamber and then unloading the single silicon wafer to the secondload-lock chamber; and

a cleaning step of vapor etching in the process chamber with a gascontaining hydrogen chloride,

wherein the cleaning step is performed before and after repeating apredetermined number of times a series of growth procedures includingthe first step, the second step, the third step, and the fourth stepunder the same conditions, and

a correspondence between a total number of nitrogen purges on thetransfer chamber throughout the series of growth procedures performedthe predetermined number of times and a maximum number of the series ofthe growth procedures is predetermined, and the predetermined number oftimes is set within the maximum number of the series of the growthprocedures.

(2) The method of producing an epitaxial silicon wafer, according to (1)above, wherein the maximum number of the series of the growth proceduresis determined based on a threshold number of times by which a backsidehaze is formed on a back surface of the epitaxial silicon wafer.

(3) The method of producing an epitaxial silicon wafer, according to (1)or (2) above, wherein in predetermining the correspondence, a nitrogenpurge gas flow rate for the transfer chamber is changed, and an exhaustvelocity for the transfer chamber is adjusted depending on the change ofthe nitrogen purge gas flow rate.

Advantageous Effect

This disclosure provides a method of producing an epitaxial siliconwafer, which is excellent in productivity and prevents the formation ofa backside haze in repeating single-wafer processing epitaxial growthprocedures on a plurality of silicon wafers without cleaning a processchamber after each epitaxial growth procedure.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic view illustrating a typical single-waferprocessing epitaxial growth apparatus:

FIG. 2 is a diagram illustrating cleaning methods for a process chamberof a single-wafer processing epitaxial growth apparatus:

FIG. 3 is a graph illustrating the relation between the number of theseries of epitaxial growth procedures and the haze value in ExperimentalExample 1:

FIG. 4 is a graph illustrating the relation between the total purgenumber for a transfer chamber and the maximum multi depo number inExperimental Example 2; and

FIG. 5 is a graph illustrating the relation between the purge number fora transfer chamber per deposition and the maximum multi depo number inExperimental Example 2.

DETAILED DESCRIPTION

Embodiments of this disclosure will now be described with reference tothe drawings. Note that only main features of the structure areschematically depicted for brevity.

(Method of Producing Epitaxial Silicon Wafer)

A method of producing an epitaxial silicon wafer according to thisembodiment is a method of producing an epitaxial silicon wafer, whichepitaxially grows a silicon epitaxial layer on a surface of a siliconwafer. Here, in describing the production method according to thisembodiment, the reference numerals for the typical single-waferprocessing epitaxial growth apparatus described above with reference toFIG. 1 are used.

The production method of this embodiment includes a first step ofloading a single silicon wafer 10 from a first load-lock chamber 100into a transfer chamber 200 kept flushed with nitrogen gas; a secondstep of loading the single silicon wafer 10 from the transfer chamber200 into a process chamber 300; a third step of forming a siliconepitaxial layer 20 on a surface of the single silicon wafer 10 in theprocess chamber 300; a fourth step of unloading the single silicon wafer10 on which the silicon epitaxial layer 20 is formed (i.e., an epitaxialsilicon wafer 1) from the process chamber 300 to the transfer chamber200 and then to a second load-lock chamber (not shown); and a cleaningstep of vapor etching in the process chamber 300 with a gas containinghydrogen chloride.

As in the multi depo process described with reference to Process B inFIG. 2, the cleaning step is performed before and after repeating apredetermined number of times a series of growth procedures includingthe first step, the second step, the third step, and the fourth stepunder the same conditions. Here, in this embodiment, the correspondencebetween the total number of nitrogen flushing operations on the transferchamber throughout the series of growth procedures performed thepredetermined number of times and the maximum number of the series ofgrowth procedures is predetermined, and the predetermined number oftimes is set within the maximum number of the series of growthprocedures.

In this embodiment, hereinafter, the process of repeating the aboveseries of growth procedures a predetermined number of times under thesame conditions is referred to as “multi depo process”, and theforegoing predetermined number of times is referred to as “multi deponumber”, and the above maximum number of the series of growth proceduresis referred to as “maximum multi depo number”. When the multi depoprocess is performed a number of times exceeding the maximum multi deponumber even once, faults such as a backside haze are caused. Further,epitaxial growth procedures of the Nth multi depo process (where N is aninteger equal to or more than 1 and less than the maximum multi deponumber) are referred to as “growth procedures of the Nth deposition”.

First, a typical single-wafer processing epitaxial growth apparatuswhich can be used in the production method according to this embodimentis described with reference to FIG. 1 mentioned above.

The single-wafer processing epitaxial growth apparatus includes a firstload-lock chamber 100, a transfer chamber 200, and a process chamber300. The first load-lock chamber 100 and the transfer chamber 200 areconnected via a gate valve 150, and the transfer chamber 200 and theprocess chamber 300 are connected via a gate valve 250. The firstload-lock chamber 100 is evacuated by a pump and purging with nitrogengas, thus a nitrogen atmosphere is maintained in the chamber. Further,the transfer chamber 200 is evacuated and purged with nitrogen gas, thusa nitrogen atmosphere is maintained in the chamber. Further, the processchamber 300 is evacuated and purged with hydrogen gas so that a hydrogenatmosphere can be maintained in the chamber. Note that since sourcegases for epitaxial growth, etching gas for cleaning, etc. areintroduced into the process chamber 300, these gases are collected in agas scrubber.

On the other hand, the first load-lock chamber 100 is also connected toa storage enclosure 400 for storing a silicon wafer 10 via a gate valve50. The first load-lock chamber 100 may receive or send the storageenclosure 400 from/to the outside of the epitaxial growth apparatus, andwhen the storage enclosure 400 is a front opening unified pod (FOUP),the first load-lock chamber 100 may receive or send a single siliconwafer 10 unloaded from the FOUP. FIG. 1 is a schematic view illustratingthe case where the silicon wafer storage enclosure 400 is a FOUP. In theexample illustrated in FIG. 1, the first load-lock chamber 100 isconnected to the storage enclosure 400 via the gate valve 50, andsilicon wafers 10 are taken out of the storage enclosure 400 one by oneby a robot (also called a factory interface: F1, for example) not shown.Here, when the storage enclosure 400 or a silicon wafer 10 from the FOUPis loaded and unloaded into/from the first load-lock chamber 100, theair flows into the load-lock chamber 100. To address this risk, thefirst load-lock chamber 100 is flushed with nitrogen that is an inertgas, thus a nitrogen atmosphere is maintained. Note that in addition tothe first load-lock chamber 100 and the storage enclosure 400 from whichsilicon wafers 10 to be subjected to deposition are taken out, a secondload-lock chamber and a storage enclosure for storing epitaxial siliconwafers 1 having been subjected to deposition are typically provided in asingle-wafer processing epitaxial growth. The first load-lock chamber100 and the second load-lock chamber have the same functions, forexample, being flushed with nitrogen, except that they have differenttransfer paths.

Note that the storage enclosure 400 used for silicon wafers, can be asilicon wafer storage enclosure used to transfer or store semiconductorwafers, which is, for example, an enclosure called a FOUP as mentionedabove or a front opening sipping box (FOSB) that is standardized by theSEMI Standards. The storage enclosure 400 can store for example 25silicon wafers 10.

A wafer transfer unit (not shown) for transferring the silicon wafers 10is placed in the transfer chamber 200. The transfer chamber 200 is alsoflushed with nitrogen which is an inert gas, to be filled with nitrogengas, thus a nitrogen atmosphere is maintained. Specifically, nitrogengas is introduced through an inlet tube 210 depicted in FIG. 1, and thegas inside the transfer chamber 200 is exhausted through an outlet tube220.

When the silicon wafers 10 to be subjected to deposition are loaded intothe process chamber 300, the silicon wafers 10 are carried one by one bythe wafer transfer unit from the first load-lock chamber 100 into thetransfer chamber 200 and from the transfer chamber 200 into the processchamber 300. Note that when the silicon wafers 10 are carried betweenthe chambers, the gate valves 150 and 250 are sequentially opened, andthe gate valves 150 and 250 are closed except when the wafers are loadedinto the chambers. Therefore, the nitrogen atmosphere of the firstload-lock chamber 100 and the hydrogen atmosphere of the process chamber300 do not come in contact with each other.

A susceptor 310 on which the silicon wafer 10 is placed is placed insidethe process chamber 300. A silicon epitaxial layer 20 can be formed byperforming epitaxial growth in the process chamber 300.

A silicon wafer 10 on which the silicon epitaxial layer 20 has beenformed (i.e., an epitaxial silicon wafer 1) is typically unloaded to thesecond load-lock chamber and the storage enclosure for storing in thereverse order to the order of the above described loading operation.

The steps of the method of producing an epitaxial silicon wafer,according to this embodiment will now be described in detail.

<First Step>

In the first step, a single silicon wafer 10 is loaded from the firstload-lock chamber 100 into the transfer chamber 200 kept flushed withnitrogen gas. When the silicon wafers 10 are loaded, the silicon wafers10 are taken out of the storage enclosure 400 one by one, or the siliconwafer storage enclosure 400 is loaded into the first load-lock chamber100 as described above. Further, when the silicon wafer 10 is loadedinto the transfer chamber 200, the gate valve 150 is opened, and thesilicon wafer 10 is loaded by the transfer unit in the transfer chamber200 from the first load-lock chamber 100 into the transfer chamber 200.After this loading, the gate valve 150 is closed. The gate valve 150 isusually opened for a time period of approximately 5 s to 15 s, and thisalso applies to the other gate valves.

<Second Step>

In the second step, the silicon wafer 10 is loaded from the transferchamber 200 into the process chamber 300. When the silicon wafer 10 isloaded, the gate valve 250 is opened, and the silicon wafers 10 areloaded from the transfer chamber 200 into the process chamber 300 by thetransfer unit in the transfer chamber 200. Here, the silicon wafer 10can be placed on the susceptor 310 in the process chamber 300 using thetransfer unit. The temperature inside the process chamber 300 when thesilicon wafer 10 is loaded into the chamber is typically around 650° C.or more and 800° C. or less.

<Third Step>

In the third step, a silicon epitaxial layer 20 is formed on a surfaceof the single silicon wafer 10 in the process chamber 300. For example,source gases such as dichlorosilane, trichlorosilane, etc. areintroduced into the process chamber 300 using hydrogen as a carrier gas,and epitaxial growth can be performed on the silicon wafer 10 by CVD attemperatures in the range of 1000° C. to 1200° C. although depending onthe source gases used. Thus, the epitaxial silicon wafer 1 can beformed.

<Fourth Step>

In the fourth step, the single silicon wafer 10 on which the siliconepitaxial layer 20 is formed (i.e., the epitaxial silicon wafer 1) isunloaded from the process chamber 300 to the transfer chamber 200 andthen to the second load-lock chamber (not shown). Specifically, theformed epitaxial silicon wafer 1 is transferred from the process chamber300 to the load-lock chamber through the transfer chamber 200 in thereverse order to the orders of the first step and the second stepdescribed above, and the epitaxial silicon wafer is stored in thestorage enclosure for storing.

As described above, through the series of growth procedures includingthe first step, the second step, the third step, and the fourth step, asingle epitaxial silicon wafer 1 can be formed using the single-waferprocessing epitaxial growth apparatus. Although depending on thethickness of the silicon epitaxial layer 20 to be formed, performing theseries of growth procedures once typically requires approximately 3 minto 5 min.

<Cleaning Step>

In the cleaning step, the inside of the process chamber 300 is vaporphase etched with a gas containing hydrogen chloride. The time duringwhich the vapor phase etching is performed can be appropriatelydetermined depending on the multi depo number. More specifically, theinside of the process chamber 300 is heated to a predeterminedtemperature, and the gas containing hydrogen chloride is delivered intothe process chamber 300, thus silicon deposited inside the processchamber 300 through epitaxial growth is vapor etched. For example, thetemperature inside the process chamber 300 may be set to 900° C. or moreand 1200° C. or less, and the gas containing hydrogen chloride may bedelivered for 10 s or more and 1500 s or less.

In this embodiment, a so-called multi depo process is performed: namely,a plurality of silicon wafers 10 are subjected to a series of epitaxialgrowth procedures which is repeated a predetermined number (multi deponumber) of times without cleaning the process chamber 300 after eachprocess. This being the case, in this embodiment, the above-mentionedcleaning step is performed before and after repeating the series ofgrowth procedures including the first step, the second step, the thirdstep, and the fourth step a predetermined number (multi depo number) oftimes under the same conditions.

<Determination of Predetermined Number of Times (Multi Depo Number)>

Here, in this embodiment, the correspondence between the total number ofnitrogen flushing operations on the transfer chamber 200 throughout theseries of growth procedures performed the predetermined number of times(hereinafter “total purge number”) and the maximum number of the seriesof growth procedures (maximum multi depo number) is predetermined, andthe predetermined number of times (multi depo number) is set within themaximum number of the series of growth procedures.

As will be described in detail below in the Examples, our study revealedthat the maximum multi depo number greatly depends on the total purgenumber. We found that no backside haze was formed on the epitaxialsilicon wafer 1 when the multi depo number was set within the maximummulti depo number. This also applies to the case where epitaxial growthconditions such as thickness are changed.

Note that the total purge number can be defined as follows. Thisdefinition is based on the assumption that the entire gas in thetransfer chamber 200 is replaced when nitrogen gas is introduced by anamount corresponding to the volume of the transfer chamber 200. First,the purge number per series of growth procedures (“purge number perdepo”) is defined by the following formula [1].

$\begin{matrix}{\left\lbrack {{Purge}\mspace{14mu} {number}\mspace{14mu} {per}\mspace{14mu} {Depo}} \right\rbrack = \frac{\left\lbrack {{Growth}\mspace{14mu} {time}\mspace{14mu} \left( \min \right)} \right\rbrack \cdot \left\lbrack \begin{matrix}{{Purge}\mspace{14mu} {flow}\mspace{14mu} {rate}\mspace{14mu} {of}\mspace{14mu} {Nitrogen}\mspace{14mu} {into}} \\{\; {{Transfer}\mspace{14mu} {chamber}\mspace{14mu} ({sim})}}\end{matrix}\; \right\rbrack}{\left\lbrack {{Volume}\mspace{14mu} {of}\mspace{14mu} {transfer}\mspace{14mu} {{chamber}(L)}} \right\rbrack}} & \lbrack 1\rbrack\end{matrix}$

Accordingly, the total purge number of purges of the transfer chamber200 with nitrogen gas after performing the multi depo process the multidepo number of times is defined by the following formula [2].

[Total purge number]=[Multi depo number]·[Purge number per Depo]  [2]

Here, the correspondence between the total purge number and the maximummulti depo number can be determined for example as follows. First, asilicon wafer of the same type as the silicon wafer 10 used in theproduction of an epitaxial silicon wafer according to this embodiment isprepared. This silicon wafer of the same type is hereinafter referred toas a test silicon wafer. An epitaxial silicon wafer is formed byperforming the multi depo process under a first set of conditions usingthe test wafer. Further, an epitaxial silicon wafer is formed byperforming the multi depo process under a second set of conditions inwhich only the purge flow rate of nitrogen into the transfer chamber ischanged from the first set of conditions. The presence or absence of abackside haze on the epitaxial silicon wafer formed under each set ofconditions was determined by observation, the threshold number of times,that is, the number of depositions causing a backside haze isdetermined, and a calibration curve can be found based on the obtainedthreshold. Instead of performing the observation, it is also preferredto measure the haze value, which is an indication of the surfaceroughness of the epitaxial silicon wafer formed. The haze value can beobtained by measurement using, for example, a particle counter SP-1manufactured by KLA-Tencor Corporation in the DWN mode.

Examples of the thus obtained calibration curve are presented in FIGS. 4and 5 to be described in Examples. The graphs demonstrated that in orderto increase the maximum multi depo number, the number of purges withnitrogen gas needed to be increased at an increasing rate. In otherwords, the purge number per depo needed to be increased.

Preferably, in predetermining the above correspondence, the flow rate ofnitrogen gas purge of the transfer chamber 200 is changed and theexhaust velocity for the transfer chamber 200 is adjusted depending onthe change in the nitrogen gas purge flow rate. This maintains a balancebetween the pressures of the transfer chamber 200 and the processchamber 300.

In order to increase the nitrogen gas purge flow rate, the pressure orthe flow rate of nitrogen gas may be increased using the inlet tube 210through which the nitrogen gas flows in. Specifically, the filterpressure drop may be reduced, or the number of inlets may be increased.

EXAMPLES

Next, in order to clarify the effects of this disclosure, examples aregiven below, however, this disclosure is not limited to the followingexamples in any way.

Experimental Example 1

First, the following experiments were performed to find the differenceof the formation of a backside haze between the first half of the multidepo process and the latter half of the multi depo process.

A silicon epitaxial wafer was produced according to the followingprocedure by the multi depo process using the epitaxial growth apparatusdepicted in FIG. 1. As a substrate for the silicon epitaxial wafer, aboron-doped silicon wafer 10 having a diameter of 300 mm was used.

<Conditions 1>

The silicon wafer 10 was loaded from the first load-lock chamber 100into the process chamber 300, followed by hydrogen bake-out, and asilicon epitaxial film was then grown at 1130° C. by 2 μm to form anepitaxial silicon wafer 1, which was unloaded to the second load-lockchamber. Thus, a series of growth procedures was performed. This seriesof growth procedures was repeated up to the fifteenth deposition. Eachdeposition took 200 s including the transfer time required to unload andload the wafer and the growth time. Trichlorosilane gas was used as araw material source gas, diborane gas as a dopant gas, and hydrogen gasas a carrier gas.

<Conditions 2>

An epitaxial silicon wafer was formed by repeating the multi depoprocess up to the fifteenth deposition under the above Conditions 1except that the nitrogen gas purge flow rate for the transfer chamber200 was increased by 18% from that of Conditions 1. Note that thepressure of the transfer chamber was adjusted proportionately with theincrease of the purge flow rate, and the balance between the pressuresof the transfer chamber 200 and the chamber 300 was maintained tosubstantially the same degree as that in Conditions 1.

The back surfaces of the epitaxial silicon wafers produced underConditions 1 and Conditions 2 were subjected to measurement using aparticle counter SP-1 manufactured by KLA-Tencor Corporation in the DWNmode to determine the haze value. The results are given in FIG. 3.

Under Conditions 1, no backside haze was observed during the first 8depositions under concentrated light, and a backside haze was observedfrom the ninth deposition. Accordingly, the maximum multi depo numberunder Conditions 1 was 8. The total number of flushing operations by anitrogen purge of the transfer chamber in the first 8 depositions underConditions 1 was calculated as 9.2 (approximately 1.2 purges perdeposition).

On the other hand, under Conditions 2, no backside haze was observedduring the first 10 depositions under concentrated light, and a backsidehaze was observed from the eleventh deposition. Accordingly, the maximummulti depo number under Conditions 2 was 10. Further, the haze value wasfound to increase sharply as the multi depo process was continued beyondthe maximum multi depo number of times. The total number of flushingoperations by a nitrogen purge of the transfer chamber in the first 10depositions under Conditions 2 was calculated as 13.7 (approximately 1.4purges per deposition).

Experimental Example 2

The correspondence between the total number of flushing operations by anitrogen purge of the transfer chamber and the maximum multi depo numberwas determined in the same manner as in Experimental Example 1 exceptthat the nitrogen gas purge flow rate for the transfer chamber 200 waschanged. The results are given in FIG. 4. FIG. 4 demonstrates that theformation of a backside haze was prevented by setting the multi deponumber depending on the total purge number.

Further, it was also found that the total number of flushing operationsby a nitrogen purge of the transfer chamber is required to be sharplyincreased in order to increase the maximum multi depo number. Although alarger number of flushing operations performed by a nitrogen purge ofthe transfer chamber is considered to reduce the effect of oxygen;however, oxygen is not readily exhausted from the transfer chamber by anitrogen purge. Conceivably, in the latter half of the multi depoprocess, adsorption of oxygen components onto the inner surface of thetransfer chamber was saturated and desorption of oxygen from the innersurface of the transfer chamber suddenly became dominant. Thus, thecorrelation between a backside haze and the purge number is consideredto have sharply changed.

As described above, the formation of a backside haze in the latter halfof the multi depo process can be prevented by predetermining thecorrespondence between the total number of purges of the transferchamber using nitrogen gas in the case of performing the multi depoprocess the multi depo number of times and the maximum multi depo numberand setting the multi depo number based on the correspondence, thus theproductivity can be improved.

Further, the relation between the maximum multi depo number and thepurge number for the transfer chamber per deposition, which is derivedfrom the graph of FIG. 4 is presented in FIG. 5. Setting the maximummulti depo number based on the purge number for the transfer chamber perdeposition can prevent the formation of a backside haze in the latterhalf of the multi depo process. On the other hand, adjusting the purgenumber for the transfer chamber per deposition (i.e., the purge flowrate of nitrogen gas in the transfer chamber) can also prevent theformation of a backside haze in the latter half of the process.

INDUSTRIAL APPLICABILITY

This disclosure provides a method of producing an epitaxial siliconwafer, which is excellent in productivity and prevents the formation ofa backside haze in consecutive single-wafer processing epitaxial growthprocedures on a plurality of silicon wafers without cleaning a processchamber after each epitaxial growth procedure.

REFERENCE SIGNS LIST

-   -   1: Epitaxial silicon wafer;    -   10: Silicon wafer;    -   20: Silicon epitaxial layer;    -   50: Gate valve;    -   100: (First) load-lock chamber;    -   150: Gate valve;    -   200: Transfer chamber;    -   210: Inlet tube;    -   220: Outlet tube;    -   250: Gate valve;    -   300: Process chamber;    -   310: Susceptor;    -   400: Storage enclosure.

1. A method of producing an epitaxial silicon wafer, which epitaxiallygrows a silicon epitaxial layer on a silicon wafer, comprising: a firststep of loading a single silicon wafer from a first load-lock chamberinto a transfer chamber kept purged with nitrogen gas; a second step ofloading the single silicon wafer from the transfer chamber into aprocess chamber; a third step of forming a silicon epitaxial layer on asurface of the single silicon wafer in the process chamber; a fourthstep of unloading the single silicon wafer on which the siliconepitaxial layer is formed, from the process chamber to the transferchamber and then unloading the single silicon wafer to the secondload-lock chamber; and a cleaning step of vapor etching in the processchamber with a gas containing hydrogen chloride, wherein the cleaningstep is performed before and after repeating a predetermined number oftimes a series of growth procedures including the first step, the secondstep, the third step, and the fourth step under the same conditions, anda correspondence between a total number of nitrogen purges on thetransfer chamber throughout the series of growth procedures performedthe predetermined number of times and a maximum number of the series ofthe growth procedures is predetermined, and the predetermined number oftimes is set within the maximum number of the series of the growthprocedures.
 2. The method of producing an epitaxial silicon wafer,according to claim 1, wherein the maximum number of the series of thegrowth procedures is determined based on a threshold number of times bywhich a backside haze is formed on a back surface of the epitaxialsilicon wafer.
 3. The method of producing an epitaxial silicon wafer,according to claim 1, wherein in predetermining the correspondence, anitrogen purge gas flow rate for the transfer chamber is changed, and anexhaust velocity for the transfer chamber is adjusted depending on thechange of the nitrogen purge gas flow rate.
 4. The method of producingan epitaxial silicon wafer, according to claim 2, wherein inpredetermining the correspondence, a nitrogen purge gas flow rate forthe transfer chamber is changed, and an exhaust velocity for thetransfer chamber is adjusted depending on the change of the nitrogenpurge gas flow rate.